1. Field of the Invention
This invention relates generally to the combustion system of a gas turbine engine and, more particularly, to a double annular combustor having concentrically disposed inner and outer annular combustors with inner and outer dome plates, each dome plate having an inner portion and an outer portion, and a cowl structure having an inner portion, an outer portion, and a middle portion, wherein the cowl outer portion is connected to the outer dome plate outer portion, the cowl inner portion is connected to the inner dome plate inner portion, and the cowl middle portion is connected to the outer dome plate inner portion and the inner dome plate outer portion.
2. Description of Related Art
Efforts to reduce emissions in gas turbine engines have brought about the use of staged combustion techniques wherein one burner or set of burners is used for low speed, low temperature conditions such as idle, and another, or additional, burner or burners are used for high temperature operating conditions. One particular configuration of such a concept is that of the double annular combustor wherein the two stages are located concentrically in a single combustor liner. Conventionally, the pilot stage section is located concentrically outside and operates under low temperature and low fuel/air ratio conditions during engine idle operation. The main stage section, which is located concentrically inside, is later fueled and cross-ignited from the pilot stage to operate at the high temperature and relatively high fuel/air ratio conditions. The swirl cups of the respective pilot and main stage sections generally lie in the same radial and circumferential planes, as exemplified by U.S. Pat. No. 4,292,801 to Wilkes, et al. and U.S. Pat. Nos. 4,374,466 and 4,249,373 to Southern.
By contrast, however, a development report to the National Aeronautics and Space Administration (NASA) on combustion system component technology for the Energy Efficient Engine (E.sup.3) discloses a double annular combustor configuration where the pilot stage (outer annular combustor) and the main stage (inner annular combustor) are radially offset (i.e., lie in distinct radial planes). U.S. Pat. No. 4,194,358 to Stenger also discloses a double annular combustor configuration where the inner and outer annular combustors are radially offset, but the pilot stage is placed in the radially inner portion of the combustor and the main stage section is placed in the radially outer portion thereof. In both the '358 patent and E.sup.3 configurations, the effective length of the main stage section is relatively short and the effective length of the pilot stage section is relatively long. This configuration allows for complete or near-complete combustion to reduce the amount of hydrocarbon and carbon monoxide emissions since there is a relatively long residence time in the pilot stage section and a relatively minimal residence time in the main stage section.
Whether the inner and outer combustors are radially aligned or not, and whether the outer annular combustor acts as the pilot stage or main stage, the prior art discloses the use of a centerbody to isolate the pilot and main stages. The intended purpose of such centerbodies is to isolate the pilot stage from the main stage in order to ensure combustion stability of the pilot stage at various operating points and to allow primary dilution air to be directed into the pilot stage reaction zone. Such centerbody designs, however, require significant cooling airflows, and can interfere with the ability of the flame to jump from the pilot stage section to the main stage section as the engine power setting is increased and both stages are required. Accordingly, the present invention proposes an alternative arrangement which eliminates the centerbody between the pilot and main stages while maintaining the desirable characteristics thereof.